The present invention relates to a multi-carrier power amplifying arrangement and to a method of controlling at least the average phase error of a multi-carrier power amplifying arrangement. The invention also relates to an array antenna with an amplifying arrangement. Particularly the invention relates to minimizing radiated power intensity for intermodulation products.
In cellular mobile communication systems the geographical coverage area of a mobile communication network is divided into cells. For each of these cells a stationary base station is arranged to communicate with a plurality of mobile stations. Each base station (BS) is connected to an antenna, or a set of antennas, covering the cell in which the BS is arranged. This means that a base station on the downlink transmits in such a manner that all mobile stations within the cell can receive the signal. Most of the transmitted information is, however, point-to-point information intended for one single mobile station only. Most of the RF power will thus be transmitted in directions in which no transceiver will receive it. If the base station could concentrate the transmitted power vertically as well as horizontally to the desired directions, using antennas with radiation patterns characterized through narrow antenna lobes, a higher efficiency could be achieved.
If the same kind of antennas are used for reception in the base station as well (on the uplink), a corresponding improvement of the reception sensitivity is obtained in the desired directions. This concentration of the output power and the reception sensitivity can be used for increasing the transmission range or for lowering the requirements on the transmitter in the base station as well as on the transmitter in the mobile station. Since the channel frequency reuse potentially can be increased with this technique, the total capacity of the mobile communication network can be enhanced. These and other advantages have in the last few years resulted in a growing interest in antenna arrays with narrow antenna beams.
It is well known in the art that all amplifiers distort an input signal. The distortion becomes greater as the power levels are increased. When the amplifier is exposed to multiple input signals, intermodulation (IM) products are introduced. These IM products must be kept low because they interfere with the system in general and with other users in particular.
Current cellular communication standards impose stringent limits on radiated power intensity for intermodulation products. The use of a central multi-carrier power amplifier (MCPA), as in current mobile communication cellular systems, suffers from a disadvantage in this respect since not only the desired signal, but also the intermodulation products will be amplified/enchanced. The consequence thereof, in combination with the power requirements on the single multi-carrier amplifier, will be that the single multi-carrier amplifier needs to employ rather sophisticated techniques to reach the desired limits on the carrier to intermodulation ratio (C/I-ratio).
Since the amount of linearisation that can be provided is technologically limited, a central amplifier has to use a significant amount of back-off to reach the desired levels of intermodulation. This leads to a decreased efficiency which in turn results in a low DC to RF conversion efficiency.
In the article xe2x80x9cApplication of Linearised Amplifiers in Adaptive Antennasxe2x80x9d by Hongxi et al, published in the IEEE MTT-S 1995 International Topical Symposium on Technologies for Wireless applications, and in which the use of adaptive antenna arrays for future mobile communication systems is discussed, a linearisation technique called feed-forward technique is proposed as the most suitable approach for suppression of intermodulation in the base stations.
The feed-forward technique, which has been applied for linearisation of a central multi-carrier power amplifier (MCPA) basically consists of two independent steps. The first step is to extract the distortion introduced by the main amplifier by comparing the amplified signal with the input signal. This step is referred to as extracting an error signal. The second step is to amplify this error signal and inject it in anti-phase and time aligned at the output of the feed-forward amplifier to thereby cancel out the distortion.
The performance of the feed-forward technique is dependent on the ability to add rotated signal vectors correctly in anti-phase and equal amplitude. This process determines how well a distortion component can be extracted or suppressed. The ability to control these variables, i.e., gain and phase, are therefore of crucial importance in feed-forward amplifier systems. Hence, phase and amplitude adapters are employed. These adapters for a given environment can be tuned to give a minimum of phase and amplitude error. However, due to, e.g., temperature drift, an imbalance may occur, resulting in insufficient intermodulation suppression. For this reason, the general feed forward linearisation method is normally combined with an adaptive phase/amplitude control.
There are two main methods of how to implement this. The first method uses the actual distortion caused by the main amplifier to control the settings of the gain and phase controls. This is possible since it equals the first step in the feed-forward process. In this case, the error signal content is minimized at the output. The second method uses a known distortion simulating signal which is injected in the amplifier path and minimized at the output, thereby also reducing the distortion introduced by the main amplifier. The term distortion shall be understood to mean any signals present in the output of a device which were not present at the input.
U.S. Pat. No. 5,386,198 discloses an example of a method of the second type for controlling a feed-forward compensated power amplifier. A spread spectrum technique is used to cover a control signal(s) and injects a composite signal at a suitable point into the feed-forward amplifier system to reduce distortion. Control signals after remapping of the spread spectrum at the output of the system are correlated in a match filter correlator and the result is used to control, in either polar or Cartesian co-ordinates, the injection, in anti-phase, of the extracted distortion into the feed forward amplifier output.
It is a drawback that the requirements as to linearisation of each amplifying means are too high and difficult, if not impossible, to meet in order to reach an acceptable carrier to intermodulation ratio (C/I-ratio).
It is particularly disadvantageous that the average phase error may be high and particularly it is disadvantageous that, when the conditions change, for example as compared to a first situation in which for a given temperature, the used control circuitry is tuned to give a zero average phase and amplitude error, the average phase error will increase considerably as for example the temperature changes (even if temperature compensated circuits are used). The average phase error will also increase considerably for high antenna gains and the limits as to the maximum allowed intermodulation intensity may easily be exceeded. It is also substantially impossible to detect and remove a resulting average phase error using the hitherto known arrangements and methods.
What is needed is therefore an amplifying arrangement, particularly a multi-carrier arrangement, through which the radiated power intensity of intermodulation products can be kept low in a simple and efficient manner. Particularly an arrangement is needed in which the requirements as to linearization are low on the respective amplifying means forming the amplifying arrangement, particularly lower than in hitherto known arrangements. An arrangement is also needed through which the power of intermodulation products can be kept low irrespectively of whether the conditions change in relation to any initial conditions, or generally under varying conditions and for a high antenna gain etc. Particularly an arrangement is needed through which at least the average phase error (and advantageously also the amplitude error) can be detected and controlled or compensated for.
Particularly an arrangement is needed which is flexible, uncomplicated and easy to fabricate. An array antenna with such an arrangement is also needed through which the abovementioned objects are achieved as well as a method of controlling an amplifying arrangement or more particularly an active array antenna.
Therefore a multi-carrier power amplifying arrangement is provided. It comprises a number of amplifying means each comprising signal input means, signal output means, a main amplifier and means for linearizing the amplifying means. Furthermore signal adding means are provided for adding the output signals from the amplifying means in phase. Error detecting means are provided for detecting the average phase error of the amplified sum signal and for providing a compensating control signal. The compensating control signal is provided to each amplifying means to compensate for the average phase error. In a particular embodiment a reference signal is provided to each amplifying means, which reference signal is used to detect the average phase error of the sum of the amplified signals in the error detecting means. Still more particularly the reference signal may consist of a pilot signal which is input to each of the amplifying means and to the error detecting means. The error detecting means consists of means for detecting the signal level of at least one component originating from the pilot signal and the result is used in providing the compensating control signal. In a particular embodiment an externally controlled signal is provided to the error detecting means to provide the compensating control signal.
In a particular embodiment the linearizing means consist of a number of feed forward loops, one for each amplifying means. Of course also other linearizing means can be used. In for example U.S. Pat. Nos. 5,051,704, 5,323,119, 5,116,634, 5,148,117, 5,148,117, 4,560,945, all of which allow external biasing, linearizing techniques are disclosed all of which in principle can be used to linearize the individual amplifying means as disclosed in the present invention.
Feed-forward loops implemented for linearization of distributed MCPA""s may decrease the non-linearities with, e.g., 30 dB. The reamining non-linear terms will have a random phase and amplitude. For any given temperature, the control circuitry can initially be tuned to yield zero average phase and amplitude error. Adaptive control circuits can be used to suppress temperature drift, but since there is a lowest detectable amplitude for the detection circuits in adaptive control circuits and since these circuits themselves are impaired with a temperature dependence, the average error may deviate from zero. This non-zero average value will be enchanced by the antenna gain and might for large antenna gains exceed the limits for allowed emitted intermodulation intensity.
Distributed multi-carrier amplifiers according to the present invention provide new possibilities of designing communication system antennas with stringent constraints on radiated intermodulation intensity. With a distributed multi-carrier power amplifier the linearization requirements on each module, or each amplifying means, are reduced as compared to the case in which a central multi-carrier power amplifier is used. The lower linearity requirements will provide a higher efficiency in the multi-carrier power amplifying arrangement due to a lower back-off in the main amplifiers. As an example, for four carriers the usual 5-8% efficiency with 30 dB C/I can be increased to about 15-20% if 20 dB C/I is allowed instead.
In a particular implementation of the invention each of the signals output from the respective amplifying means, i.e. the amplified signals, are provided to (at least one) antenna element, which antenna elements are arranged to form an array antenna. The antenna elements may be arranged in a linear array or alternatively they may be arranged in a two-dimensional array, usually planar.
As to the compensating control signal it is particularly a signal with an externally controlled phase, even more particularly, the externally controlled signal is provided to the error detecting means via a digitally controlled phase shifter. This is however not necessary and particularly the phase shifter does not have to be digitally controlled.
In one particular embodiment the entire arrangement may be realized as an RF-ASIC (Radio Frequency-Application Specific Integrated Circuit).
According to different implementations, in case a reference signal is used or even more particularly a pilot signal, such signal may be injected either before the main amplifier of each amplifying means or after the main amplifier of each amplifying means.
In a particular implementation the error detecting means includes a phase detector and an amplitude detector.
For an antenna in a matrix form the antenna elements are particularly arranged in m rows and n columns and for the antenna elements a number of amplifying means are arranged, e.g. in a similar manner. The signal output from an amplifying arrangement can be provided to one or more antenna elements.
The adding means particularly include n/m first adding means in which the output signals from the amplifying means are added in phase columnwise/rowwise and in a Butler matrix, here denoted a, second Butler matrix for reasons as will be explained below, the sum signals from the m or n first adding means respectively are added. In such an embodiment a first switching means and a first Butler matrix are provided for selecting an input beam with one of a number of different phases, for each phase a number of amplifying means and antenna elements being provided. To the second Butler matrix as referred to above a second switching means is provided for selecting output beam. In a particular implementation of the embodiment as described above, means are provided for finding the minimum of the signal output from the second switching means. Said minimum is particularly found through using varying external control signals and the result is used to provide the compensating control signal which in turn is provided to each of the amplifying means as discussed earlier.
Therefore is also an array antenna provided which includes a number of antenna elements, to which antenna elements a number of amplifying means are provided, in which an input signal is amplified and provided to one or more antenna elements. Each of the amplifying means include a main amplifier and means for linearizing the amplifying means respectively. Signal adding means are provided for adding the amplified signals output from the amplifying means in phase (coherently) and error detecting means are provided for detecting the average phase error (and amplitude error) of the amplfied signals and for providing a compensating control signal. The compensating control signal is provided to each one of the amplifying means to control/minimize the average phase error of the amplified signals input to the antenna means. In an advantageous implementation a pilot signal is input to each amplifying means which then is detected in the error detecting means to provide an estimation of the average phase error of the sum of the amplified signals. Particularly the signal level of the, or at least one, component originating from the pilot signal output from the amplifying means is detected. The pilot signal may be (a) specific frequency component(s) or alternatively it can be a spread spectrum or CDMAxe2x80x94(Code Division Multiple Access) coded signal. In a particular implementation a signal with an externally controlled phase is provided to the error detecting means to provide the compensating control signal. The antenna elements may be arranged in a linear array or in a two dimensional array. Amplifying means are arranged in m rows and n columns. At least as many antenna elements as amplifying means are provided, e.g. in a similar manner. For each column or for each row adding means are provided in which the amplified signals are added vertically or horizontally. The resulting sum signals from either thereof are then added together in a Butler matrix. A switch is advantageously provided to select one of the beams output from the Butler matrix. Furthermore means are provided for finding the minimum of the selected output beam using a varying external control signal. The resulting average value is used to control the amplifying means.
Therefore also a method is provided for controlling at least the average phase error of a multi-carrier power amplifying arrangement for an array antenna. The amplifying arrangement includes a number of amplifying means, each providing an amplified signal to at least one antenna element of the array antenna. The method includes the steps of: providing an input signal to each of the amplifying means, adding the amplfied signals output from the respective amplifying means to generate a number of sum signals, detecting the averge phase error of the sum signal(s), and using the detected average phase error to provide a compensating control signal. The compensating control signal is then provided to each amplifying means to control/minimize at least the average phase. error. Advantageously is also the amplitude error compensated for. In a particular implementation the array antenna comprises a two dimensional array antenna and the output signals from the amplifying means are added in a number of adding means, one for each column, vertically, whereafter a Butler matrix is used in which the vertically added sum signals are added. A signal output from the Butler matrix is selected with switching means. All signals are added to find an average value, which is a scalar (real or complex) value.